1. Technical Field of the Invention
The present invention relates to a screw compressor suitable for use in a device, such as an air conditioner, a chiller unit, or a refrigerator, that forms a refrigeration cycle and a chiller unit using same.
2. Description of the Related Arts
In a case where a screw compressor is used for, for example, an air conditioner or a chiller unit, it is used with suction pressure and discharge pressure in a wide range, thus resulting in possibility that pressure in a tooth groove of a screw rotor (pressure of a compression work chamber) becomes higher than discharge pressure under some operation conditions (hereinafter referred to as over-compression). Thus, a screw compressor for reducing over-compression is suggested (for example, see Japanese Patent Application Laid-open No. S61-79886).
The screw compressor described in the Japanese Patent Application Laid-open No. S61-79886 includes: a male rotor (main rotor) and a female rotor (subordinate rotor) rotating while engaging with each other with rotation axes thereof in substantially parallel to each other; bores storing tooth parts of the male rotor and the female rotor; a main casing (housing) having an end surface opening on a discharge side of the bores in a rotor axial direction; and a discharge casing (housing wall) connected to the discharge side of the main casing in the rotor axial direction. The discharge casing has: a discharge side end surface abutting the end surface of the main casing to cover the opening of the bores; an outlet port (discharge window) formed at this discharge side end surface; a discharge chamber where compressed gas is discharged via the outlet port from the compression work chamber formed at tooth grooves of the male rotor and the female rotor; a valve hole opening near the outlet port on the discharge side end surface to at least one of a male rotor side and a female rotor side at a position opposite to a rotor rotation direction; and a bypass flow path having the valve hole and the discharge chamber communicate with each other, and the discharge casing is provided with a valve device (overflow valve) opening and closing the valve hole.
The valve device has: a valve body arranged in the valve hole; and a spring (press spring) biasing the valve body to a main casing side. Then for example, in a case where the valve body is moved to the main casing side to close the valve body, compressed gas is discharged from the compression work chamber to the discharge chamber via the outlet port. On the other hand, in a case where the valve body is moved oppositely to the main casing side to open the valve body, the compressed gas is discharged to the discharge chamber not only via the outlet port but also via the valve hole and the bypass flow path. This reduces over-compression.
As a stopper of the valve body, a step part is formed at the valve body and the valve hole. Consequently, for example, in a case where the valve body has moved to the main casing side, an apical surface of the valve body is on the same plane with respect to the end surface of the discharge casing, which prevents the valve body from contacting with a tooth part end surface of the rotor.
However, it has been found that the following problems need to be improved for the conventional air described above.
Specifically, in the conventional art, pressure from the compression work chamber is acting on the valve body, and thus the compression work chamber turns into an excessively compressed state (pressure of the compression work chamber>pressure of the discharge chamber (discharge pressure), and if it defeats press force of the spring, the valve body is opened. However, when the valve body has opened, pressure of the valve body on a compression work chamber side immediately becomes equal to pressure on a discharge chamber side. On the other hand, back pressure of the valve body is always the pressure of the discharge chamber, and thus pressure acting on the valve body is immediately balanced. Thus, due to the action of the spring biasing the valve body to the main casing side, the valve body is immediately closed. Therefore, in a case where the compression work chamber has turned into the excessively compressed state, the valve body repeats opening and closing at every passage of the compression work chamber through the valve body following rotor rotation, posing a problem that hit sound or vibration caused by hitting the stopper with the valve body occurs.